Update variables.html (#218)

Period for good grammar.

lib/init/variables.html

@@ -394,7 +394,7 @@ PERMANENT
   <dd>tells the display library which driver to use; possible
   values: <em><a href="cairodriver.html">cairo</a></em>, <em><a href="pngdriver.html">png</a></em>, <em><a href="psdriver.html">ps</a></em>,
     <em><a href="htmldriver.html">html</a></em>
-    or <em>default</em><br>Default display driver
+    or <em>default</em>.<br>Default display driver
     is <em><a href="cairodriver.html">cairo</a></em> (if available)
     otherwise <em><a href="pngdriver.html">png</a></em>.
   </dd>